— 
ADDRESS. UY, 
Poincaré concur in holding that disturbance will ultimately creep into 
any system of bodies moving even in so-called stable orbits ; and this is 
so even apart from the resistance offered to the moving bodies by any 
residual gas there may be scattered through space, The stability is 
therefore only relative, and a planetary system contains the seeds of its 
own destruction. But this ultimate fate need not disturb us either 
practically or theoretically, for the solar system contains in itself other 
seeds of decay which will probably bear fruit long before the occurrence 
of any serious disturbance of the kind of which I speak. 
Before passing on to a new topic I wish to pay a tribute to the men 
to whom we owe the recent great advances in theoretical dynamical 
astronomy. As treated by the master-hands of Lagrange and Laplace 
_ and their successors, this branch of science hardly seemed to afford scope 
for any great new departure. But that there is always room for dis- 
covery, even in the most frequented paths of knowledge, was illustrated 
when, nearly thirty years ago, Hill of Washington proposed a new 
method of treating the theory of the moon’s motion in a series of papers 
which have become classical. I have not time to speak of the enormous 
labour and great skill involved in the completion of Hill’s Lunar Theory, 
by Ernest Brown, whom I am glad to number amongst my pupils and 
friends ; for I must confine myself to other aspects of Hill’s work. 
The title of Hill’s most fundamental paper, namely, ‘On Part of the 
Motion of the Lunar Perigee,’ is almost comic in its modesty, for who 
would suspect that it contains the essential points involved in the deter- 
mination of perpetual orbits and their stability? Probably Hill himself 
did not fully realise at the time the full importance of what he had done, 
Fortunately he was followed by Poincaré, who not only saw its full meaning 
but devoted his incomparable mathematical powers to the full theoretical 
development of the point of view I have been laying before you. 
Other mathematicians have also made contributions to this line of 
investigation, amongst whom I may number my friend Mr. Hough, 
chief assistant at the Royal Observatory of Cape Town, and myself. But 
without the work of our two great forerunners we should still be in utter 
darkness, and it would have been impossible to give even this. slight 
sketch of a great subject. 
The theory which I have now explained points to the origin of the 
sun and planets from gradual accretions of meteoric stones, and it 
makes no claim to carry the story back behind the time when there was 
already a central condensation or sun about which there circled another 
condensation or planet. Butmore than a century ago an attempt had 
already been made to reconstruct the history back to a yet remoter past, 
and, as we shall see, this attempt was based upon quite a different sup- 
position as to the constitution of the primitive solar system. I myself 
believe that the theory I have just explained, as well as that to which I 
am coming, contains essential elements of truth, and that the apparent 
discordances will some day be reconciled. The theory of which I speak 
1905, 0 
